1. Field of the Invention
This invention relates to an image forming apparatus and an auto-illuminating controller thereof and, more particularly, to an apparatus for performing illumination amount control in accordance with the image density of an original, in an image scanner such as a laser printer or in a analog or digital copying machine having an image scanner and performing an electrophotogaphic process.
2. Description of the Related Art
In recent times, an apparatus for automatically controlling the illumination amount in illuminating scanning, in accordance with an original image, has been developed. The apparatus has been put into practice in, for example, a copying machine in which a scanner is moved reciprocally with respect to the fixed original, to scan the original. The light reflected by the original is guided onto a charged photosensitive body, through an optical system, to form, on the photosensitive body, an electrostatic latent image corresponding to the original image. The latent image is developed, and the developed image is transferred onto paper.
According to a known technique for illumination control, a light-receiving element is located in a path of the light reflected by an original. This element receives part of the light reflected by the original and converts it into an electrical signal. From this signal, the density of the image on the original is detected. From the detected image density it is determined how much light must be applied to the original, thereby to scan the original appropriately. If the light-receiving element is positioned near the original or a photosensitive body on which the reflected light is focused, it would be very difficult, if not impossible, for a single light-receiving element to detect the density of the original image across its entire width. Thus, in order to detect the density of the entire surface of the original, the element is fixed near an image-forming element located within an optical system, and outside the optical axis of the optical system.
However, even in the case of the above-described apparatus, uniform detection throughout the entire width of the original is not necessarily achieved. Rather, the sensitivity of the light-receiving element tends to be higher near a central portion of the original, in the widthwise direction, due to the position of the element. If a portion of the original has an extremely high density, the scanner is controlled to apply more light on this portion than on the other portions of the original. As a result, that portion of the copy which corresponds to this high-density portion of the original has a density lower than the density of the said original portion. The density of this copy portion may be so low that the data copied on this portion can hardly be legible.
The apparatus is disadvantageous in another aspect. When only a desired portion of the original is copied, the auto-illuminating sensor adjusts the amount of light to be applied to the desired portion is adjusted in accordance with the density of the sub-portion of the desired portion, which is denser than another other sub-portion. Consequently, those portions of the copied image which correspond to the lens dense sub-portions will have so low a density that they are almost illegible.
In view of this, it can be regarded as impossible to accomplish appropriate illuminating control by means of conventional techniques.